Operation of glow discharges



Nov. 3, 1959 H. KNUPPEL 2,911,570

OPERATION OF GLOW DISCHARGES Filed May 2, 1957 5 Sheet-Sheet 1 He/muf Kni/ppe/ Afforneys 5 Sheets-Sheet 3 Filed May 2, 1957 Afforneys 5 Sheets-Sheet 4 Filed May 2, 1957 He/muf Knapp/e MWgZM Affomeys Nov. 3, 1959 H. KNUPPEL 2, 7

OPERATION OF GLOW DISCHARGES Filed May 2, 1957 5 Sheets-Sheet 5 He/muf K/u'ippe/ Attorneys a 2,911,570 OPERATION or GLOW DISCHARGES Helmut Kniippel, Dortr'nund-Lottringhausen, Germany, assignor to Dortmund-Hinder Hiittenunion Aktiengesellschaft, Dortmund, Germany Application May 2, 1957, SerialNo; 656,629 Claims priority, application Germany May 4,1956 12 Claims. (or. 315-325 This invention relates to improvements in apparatus for operating a glow discharge for industrial purposes.

The glow discharge is a well-known form of electric discharge between a pair of electrodes-disposed in a partly evacuated tube 'and connected to a voltage source. For a considerable time, glow discharge tubes have been used extensively for illumination purposes, tubes for such purposes commonly being called glow lamps, Glow or it maybe used for the purpose of initiating or effecting chemical reactions, or of treating the surfaces of metal objects. This invention is only related to glow discharges for applications of the latter kind, as distinguished from glow lamps, stabilizers and related devices. One major difference between thetwo kinds of utilizing glow discharges just referred to lies in the fact that glow discharges for the said industrial purposes are usually. operated at currents of an order of .magnitude between 5 and several hundreds of 'amperes. Also, while in the first mentioned low-power devices the evacuated chamber is usually a glass tube of comparatively small dimensions, industrial applications of the glow discharge require vacuum chambers made of steel, chambers which, with regard to shape and size, are comparable to furnaces.

In utilizing a glow discharge'for say the purpose of surface treatment, the object to be treated is placed into a vacuum chamber, hereafter referred to as the vessel. The vessel includes suitable means for supporting the object, and it is equipped with at least two electrodes to which a voltage may be applied through an external circuit. Sometimes, the vessel itself may formone of the electrodes. After the object has been placed into the vessel, the latter is closed'an'd sealed, so that thereupon it may be evacuated until the internal pressure drops to a value at which a glow discharge may be maintained between the electrodes.

It is necessary for most purposes to treat the object in the atmosphere of a gas dilferent from aha For exglow discharges have also been found ired States-Patent ()fiFice.

"for the purpose under discussion,

2,91 1,570 Patented Nov. 3, 1959 vessel, a voltage of suitable magnitudeis, applied to the glowdischarge is ignited between electrodes so that a the electrodes.

Ignition of the glowfdischarge may be achieved just by raising the voltage sufficiently above the value which is required for subsequently maintaining the discharge. Sometimes, however, the difference between the voltage value required for ignition and the lower voltage value sufiicient for maintaining the discharge is undesirably large. In such cases, an auxiliary electrode may be provided, often referred to as. ignition electrode.

For operating the glow discharge, either D.-C.'or A.-C. may be-used. It was found, however, that D.-C. is preferable to A.-C. in many respects, the reason being that the most elfective region within a glow discharge .is that usually referred to as cathode drop, which region will at all times lie directly adjacent to theobject if D.-C. is used and the object is connected to operate as cathode.

The major elements of the external circuit usually comprise a source of electric "energy and an impedance, both; connected in series with'each other and with the vessel. It D.-C. is used, the object is often connected directly or indirectly to the negative terminal of the source, thus becoming the cathode, which is done for the reasons explained above.

Glow discharges have the Well-known tendency to.

break down to an arc: discharge. Such a breakdown is particularly undesirable'in theme of glow discharges because an arc discharge may have adisturbing or even detrimental effect ample, 'nitriding may be carried .out .in an atmosphere of ammonia, while a gas suitable for carbonization is methane. In such cases the vessel is connected .by one pipe to a vacuum pump and by another pipe tof 'a gas supply, the latter pipe comprising a throttle. While the some gas from. the. supply with only anegligible percentage of air remaining. After upon the chemical reactions which is to initiate or to affect, and it usually has a damaging eflectupon an,articleunder treatment or upon equipment disposed inthe vessel, if the arc is sustained for any appreciable time. To operate a glow discharge does not present any major difficulties if the discharge current is comparatively low, a simple resistor connected in series with the vessel then being a sufficient means to eliminate anydanger of such breakdowns occurring. -This is because in the lowercurrent range a breakdown of the glow discharge to an arc discharge is impossible if the current is prevented fromincreasing as far as is required for igniting an arc, the arc discharge curent being greater than the glow discharge current in the. said range. However, if a glow discharge is operated at a higher current this simple means current equal to or even less than to sustain the glow discharge.

The border value separating what has been referred to here as the lower and higher current ranges depends to some extent upon circumstantial conditions, such as the nature of the gas, the gas pressure, the voltage, the configurationof the electrodes etc. However, it is safe to say that a simple series resistor will in all probability fail to operate as a breakdown preventing means if the current exceeds the order of magnitude of one ampere.

An arc discharge insidethe vesselmay be tolerated if the arc is allowed to burn for a short period of time only, the length of this time depending upon the type of treatment but, in any event, rarely exceeding the order of magnitude of one tenth ofa second. An are burning the current required fora limited time -is foundto do no harm to the object the arc discharge ispresent may be achieved, for examplefby v providing in the circuit an automatic breaker which is sensitive. to the increase in current occurring when the glow discharge breaks down to an arc discharge.

the glow discharge fails, because in the. higher current range an arc discharge may exist at-- a" It-isalsopossibleto providemeans by which' the circuit is closed automaticallyafter the arc has been extinguished due to the action of the breaker. The switching means in the circuit may be of the electromechanical or the electronic type. However; XtiIlCtiOllzOf the arc andzreignition, of the glow dischargezmay also .be' effected with.-

Out any electromechanical or electronic.switchingaction. if, for example, a circuit is used as it is described in: the

co-pending United States application for-patent by Fritz Harders, Helmut Kniippel, and Karl B'rotzmann, Serial No. 515,426, filed June 14, 1955, now Patent No. 2,853,- 655, and co-pending United'States application for patent by Fritz Harders, Helmut Kniippel, and Karl Brotzmann, Serial No. 634,232, filed. January 15,1957, now Patent No. 2,852,721, which latter application is a continuation in part of the first named application, Serial No. 515,426.

process of extinguishingthe arc does'notin itself present 1 any major problems. It is onlyin combination with the additional requirement of re-establishing theglow-discharge that certain difficulties arise.

[It-.is therefore one objectof'the invention to provide means allowing the operation of 'a glow discharge of high power without any substantial damage being caused to.the article under treatment,-or other parts or equipment inside the vessel, or any undesirable or-detrimental effect upon'other kinds of chemicalreactions carried out in the vessel, by a possible breakdown of the glow discharge to an arc-discharge. i

It is another object of the invention to provide means for automatically extinguishing a-possible arc discharge withina limited time after the breakdown of the glow discharge to an arc discharge has occurred, and for automatically re-igniting the glow discharge thereafter.

It is a further object of the invention to accomplish the removal of the discharge and the re-instating of the glow discharge by electronic circuit elements.

Other objects of-the invention will become apparent fromthe following description of various embodiments of the invention, illustrated in the drawings.

In-accordance with the invention, extinction of an arc discharge is accomplished by automatically firing a gas discharge-in a gas-filled bulb which; for example, may be agas triode of any one of the types known as ignitron, senditron and thyratron. This triode forms part of a circuit loop also comprising the discharge vessel. Re-ignition of the glow discharge in the said vessel is then accomplished by extinguishing the discharge in the said gas triode, as will be explained in greater detail in what follows.

In the drawings Fig. 1 is a cross-section of'a discharge vessel suitable for surface-treating metal objects and for other purposes,

Fig. 2 is a diagram illustrating the pipe connections and electrical connections made to the vessel shown in Fig. 1, i

Fig. 3 is a circuit diagram showing a more general form of circuit embodying the invention,

Fig. 4 is a circuit diagram illustrating an embodiment of the invention inmore detail,

Fig. 5 is a circuit'diagram illustrating another embodiment of the invention,

Fig. 6 is circuit diagram illustrating a further embodiment of the invention, and j 1 Fig. 7 is a plot vs. the time .of the discharge-current 1n and the discharge voltage across a discharge vessel in a system according to the'invention.

Referring now to Fig. '1,;a vessel suitable for surfacetreating a metal object in-the presenceofaglow discharge.

may consist of a cylindricahcontainerlO having a lid.11 whlch- 'may befa'stenedto aflange" 12 of the container by bolts 13, a gasket 41 14 serving to seal the interior of the container. The object shownis a hollow cylinderof steel, and it is assumed that the inner surface 16 of this object is to be carbonized or nitrided. The cylinder 15 is supported by means of a bracket comprising a flange 17, a conical portion 18 and a rod 19 to which the cylinder 15 is fixed at 20. The conical portion 18 extends through an opening 21 of .conical shape in sucha way that there is 'a'narrow'gap 22formed between the two conical surfaces- The-flange 17 is held in' place by sev-- eral-. clamps'.:23:with :bolts 24,:only one clamp and bolt being shown, with a pair of gasketsZS 'and 26-of insulation material serving'as sealing means.

A metal rod 27 is arranged coaxially to and inside the object .15,.and::is 'supportedby a bracket which-extends through the sidewall of the vessel and consists of a metal rod 28 covered by a layer 29 "of insulation material. Another bracket of similar configuration and thus comprising a rod 30 and an insulation covering 31 carries a metal pin 32 fixed to its inner end. Sleeves 33 and 34 are provided for sealing.

Two pipes,'designated 35 and 36, connected to openings '37 and 38 in the side wall, are provided for evacuating the vessel and filling it with gas, as shown in Fig. 2.

The exhaust pipe connects the vessel with a vacuum pump 39 driven by a motor 40. The supply pipe 36 is con-- nected to a gas bottle 41 filled with whatever gas is to be 'used' in the process. Each pipe is equipped with one valve, 42 and 43, valve 43 also serving as an adjustable throttle to control the pressure at which the gas enters the vessel. The/pressure inside-the vessel may be read from a pressure gauge 44.

Operation of the system will now be described first with no regard to the possibility of the glow discharge breaking down to an arc discharge. After thelid 11 with the object 15 fixed thereto has been fastened to the cylinder-10,'pump 39 is set in operation with valve 42 being open and valve 43'adjusted to exert a certain amountv of throttle action. This will cause the pressure inside the vessel to drop below atmospheric pressure, the pressure value finally reached in this way depending upon the adjustment of throttle 43. Also, while the vessel is being evacuated the amount ofair contained in it will decrease. gradually whereas the percentage of gas will increase. If the pump is held in operation for a sufficient length of time, the percentage of air inside the vessel will have become negligible.

In theph'ase of operation succeeding this evacuation the pressure inside the vessel must be held within cer-' tainlimits. In some applications, the margin set by these limits is relatively wide. In such cases, operation of the pump may be discontinued and the valve 42 closed. If the pressure then approaches the upper limit of the permissible pressure range before the process is completed the pump may again be operated with the valve 42 open, to lower the pressure sufficiently.

In cases where theaforesaid margin is relatively narrow the pump maybe held in operation until the process to be described now is completed.

Let a D.-C. voltage be connected tosuitable terminals 45 and 46 on flange 17 and rod 28 resp. in such a way that the object 15, being,conductivelyconnected to flange 17, assumes a negative potential relative to rod 27, the

latter being conductively connected to rod 28 If the ticles, will be incorporated into the object within the saidv region- In this'way, carbonizing, nitriding or other treatment or processes may be accomplished. Two examples will now be. given, with numerical values that were found suitable per square centimeter referred to thesurface to be treated,

which current density, according to experience, corresponds to a voltage of about 800 volts between cathode and anode. An averagetreatment yielding a satisfactory product will require about 1 hour.

Example II sponds to a voltage of about 600 volts between cathode and anode. An average treatment yielding a satisfactory product will require about 5 hours.

Although the vessel shown in Fig. 1 is equipped for carrying out surface treatments, it will be obvious that the same type of vessel may be used for example in cases where the glow discharge serves the purpose of influencing, initiating or effecting chemical reactions between components of the gas filling.

Referring now to Figs. 3 to 6, showing several embodiments of the invention in the schematic form of wiring diagrams, it will be seen belowthat, inorder to carry out certain switching operations, I make use of electronic switches or relays of the type in which the-circuit V to be switched is closed by firing a gas discharge in a gas-filled tube and opened by extinguishing that same discharge. Such tubes are well-known to those skilled in the art, the most common types of such tubes being called thyratron, ignitron and senditron. It will be recalled that the thyratron is a gas triode having a continuously emitting thermionic cathode, an anode and a grid disposed between the cathode and. the anode, the discharge being fired by raising the grid potential above a critical value, and being extinguished by lowering the anode potential under another critical value. As distinguished herefrom, the gas triode called ignitron has a mercury pool cathode opposed to an anode, the third electrode consisting of a rod made of semiconducting material and having a tip which is immersed in the mercury pool. The ignitron is fired by letting a current exceeding a predetermined value flow in the rod and into the pool, the rod therefore often being referred to as igniter. The senditron is a tube somewhat similar to the ignitron except for the fact that both the anode and the cathode consist of mercury, and that the third electrode or igniter acts electrostatically, the discharge being fired by applying to the igniter a rather high voltage. As in the thyratron, the discharge is extinguished in both the ignitron and the senditron by lowering the anode potential under some predetermined value. It might be mentioned that the type of discharge in these tubes is that of a-mercury vapor arc discharge.

All three types of gas-filled tubes just briefly explained may be overloaded fora limited time; however, the amount of permissible overload is considerably greater in the ignitron and the senditron than in the thyratron. If fora given time the discharge current in --a thyratron may be raised to say 6 times normal load, the discharge current in an ignitron may be increased for the same time to between 100 and 1000 times normal current, the sendi tron then being capable of standing without damage an overload about 3000 times normal load. On the other hand, it is an advantage of the thyratron that firing requires a comparatively low voltage and a small amount of energy. p

Detailed information on the properties of gas-filled tubes of the type referred to, as well as on the related circuitry,

I the potential of anode tron. Durin'gthe subsequent transient the anode 'poten- 'tem according to the invention,

. of the arc.

, chusetts Institute of Technology, New York 1952.

In the symbolism used in the diagrams showing in Figs. 3 to 6, the thermionically emitting cathode of the thyratron is indicated by a dot, whereas the mercury pool cathode of an ignitron or a senditron is shown as a shaded segment.

Turning now to Fig. 3, a discharge vessel 50, which may be of the type shown in Figs. 1 and 2, having a cathode 51, an anode 52 and an ignition electrode 53, is shown connected to a source 54 of direct-current electric energy whose negative terminal 55 is directly connected to the cathode 51, while a lead 56 connecting. the positive terminal 57 to the anode resistor 58. 59is an ignitron whose cathode 60 is connected to cathode 51 and whose anode 61 is connected to anode 52 of vessel 50. The ignitor 62 of ignitron 59 is connected to cathode 60 across a D.-C. source 63 and a switch 64 which is normally open. The ignition electrode 53, which may be of the type of the metal pin 32 shown in Fig. 1, is connected to anode 52 across a resistor 65 and a source 66 of electric energy which may either supply D.-C. or medium frequency A.-C. Connected in parallel to resistor 58 is an impedance consisting of a resistor 67 in series with a capacitor 68. A switch 69, normally open, is connected to terminal 55 ,and to a node 70 between resistor 67 and capacitor 68. Terminal 55 may be assumed to be grounded as indicated at 71.

Fig. 3 is a largely simplified presentation of a sysits purpose being to illustrate the basic functioning of the system without regard to details, such details being shown in Figs, 4 to 6, to be explained below. a

At normal operation, with a glow discharge present in vessel 50, switches 64 and 69 are open. and the ignitron 59 is dead. With no current flowing in resistor 67, the potential of node 70 equals that of terminal 57. Due to the voltage drop across resistor 58 the potential of anodes 52 and 61 is somewhat less than that of terminal 57, the difference equalling the voltage to which capacitor 68 is charged.

Assume now that the glow discharge in vessel 50 breaks down to an arc discharge and that immediately upon this occurrence switch 64 is closed. The current generated by source 63 and flowing in ignitor 62 will then fire the ignitron 59, which establishes a short circuit across vessel 52, causing the immediate extinction The term short circuit is of course to be understood cum grano salis hereinasmuch as the ignitron when fired naturally has some apparent resistance. More accurately speaking, the ignitron when fired sets the voltage between the electrodes 51 and 52 to its own discharge voltage which, being in the order of20 volts, is just about one third or one half the voltage at which an arc can exist in the discharge space between electrodes 51 and 53. g

The time necessary to fire the ignitron 59 is extremely short. Therefore, switch 64 need not be held closed for any appreciable amount of time. The ignitron 59 is then allowed to conduct for a short time whereupon switch 69 is closed. 7 Closing switch 69 sets the potential of node '70 to ground potential, or zero potential. With the initial charge of capacitor 68 still present, the potential of ignitron anode 61 is less than the potential of node 70 and, upon closing switch 69, instantaneously assumes a value which is negative with respect to terminal 55 and cathode 60. This instantaneous dmp of 61 causes extinction of the ignitial increases exponentially and approaches the poten- 52 comprises a tial of terminal-57. At zero current.the voltage drop in" resistor 58?equals.zero, anode assuming a. potential'which exceeds the valuenecessary to sustain a glow dischargein vessel, "and, if the resistance of resistor 58is suitably dimensioned, causes re-ignitionof the glow discharge.

The probability that the discharge in vessel 50' upon ignitionre-appears in the form of an arc discharge is the greater, the greater the difference between the voltage across the vessel 50 at normal operation and the voltage at whichthe glow discharge is ignited. Also, a large difference between these voltages requires a correspondingly large resistance of resistor 58,- which is undesirable. with regard 'to the energy dissipatedin that resistor. It'is for thesereasons that I'often prefer to provide an ignitron electrode. such as electrode 53; While such an' electrode may simply be connected to the anode across resistor 65, 'I prefer to place a source such as source 66 in series with resistor 65, this source supplying a voltage sufficient for permanently maintaining a lowcurrent glow. discharge between cathode'51' and electrode 53L Resistor limits the energy of this particular discharge sufficiently to prevent it from breaking down to an arc.

It will now be shown in connection with Fig. 4 how automatic firing and'extinguishing the ignitron as well as automatic re-ignition of the glow discharge in the vessel may be obtained. Some of the circuit elements shown inFig; 3 'also appear in Fig, 4 and will therefore be described briefiy only.

The discharge vesseldesignated 72 in Fig. 4 has a cathode 73; an anode. 74 and an ignition electrode 75, the'latter being connected to the anode74 across a re sistor 76 and a source 7.7. The leads 78, 79- connecting-the electrodes 73 and 74 withthe terminals 80 and 81 of the main source 82'comprise resistors 83 and 84.

.A'n-ignitron'85 having. an anode .86, a cathode 87 and an igniter 88 is connectedin parallel'with the vessel 72 as in Fig. 3, except that the lead 89 connecting anode 86 to anode 74' comprises a resistor 90 and may be broken by the contact'system 91'of a relay 92, the contacts of that system being held closed by gravity or spring action, notshown, as long as the relay coil 93'is'not energized.

In Fig. 4, a thyratron 94'serves as a switch for igniting theignitron 87. The thyratron cathode 95 is connected to the ignitor 88, while the thyratron anode 96 is connected to terminal 81 of source 82 across resistors 97land 98, resistor 97 having 'a rather low resistance and resistor 98-having a rather highresistance. A node 99 between resistors 97 and 98 is connected to ground 100 across a capacitorltll. A simple two-loop network 102, 103is connected in parallel to resistor 83, the network comprising a capacitor-104, common to both loops 102 and 103, a resistor 105 and a capacitor 106 in loop 102 and a resistor 107 in loop 103, a voltage source 108 providing a DL-C. bias voltage being connected in series with resistor 107. The. grid 109 of thyratron 94 is connected across a resistor 110 tothe the one plate of capacitor 104 .which is not grounded.

Coil 93' of relay 92' forms part of the loop 111 of a three-loop network 111; 112, 113. In this network, the common element of loops 111 and 112 is a capacitor 114 whileloops 1121and 113 have a resistor 115'in common. Loop '113'als'o comprises resistor 90, a capacitor 116 and a resistor 117. The purpose of the networks designated 102, 103 and 111, 112, 113 is to provide time delay. The delaying effect of such networks is well-known to those familiarvvith the theory of linear networks and will therefore not be explained here in detail, full information being available from such textbooks as Communication Network's by Ernst A. Guill'emin, John Wiley and Sons, New York 1952. 7

At normal operation, with the constant discharge current'of a glow discharge in vessel 72' flowing in resistor 83; both ignitron STandthyratrori. 94' are dead, the bias 52' of vessel .50 thus 1 voltage of source 108 keeping the potential of grid 109 below the firing potential. If the glow discharge breaks down toan arc. discharge, the discharge current and the proportional voltage drop in resistor 33 abruptly'increase;

the latter-initiating-a voltage impulse appearing acrossresistor 107. This impulse increases the potentialof" grid 109 'suficiently to fire the thyratron 94." However, due to the retarding action of network 102, 103-the impulse is'delayed by a time interval depending upon'the'r time constants of the network. Firing of the thyratron 94 causes capacitor 101, charged to a voltage equalling that 1- of source 32, to discharge itself through the thyratron,

the discharge current flowing in ignitor 88 and thus firing the ignitron 85. The resistance of resistor 98 is sufficiently high'to let the potential of the anode 96 of thyratron 94' drop to a-value at which the thyratron is extinguished.

The ignitron when fired short-circuits the vessel 72' and causes extinction of the arc discharge as has been described in connection with Fig. 3. The ignitron discharge current results in a voltage drop across resistor'90'whichinturn causes a current to build up in coil 93 of relay 92 at a rate depending upon the time constants of network 111, 112, 113. The contact system 91 ofrelay 92 then breaks the ignitron current as soon as the current in coil 93 has reached a value sufi'icient to overcome the gravity or spring force acting against the magnetic force of the relay coil. Hence, the ignitron S5'is extinguished'after it has been conducting for a time interval depending upon the magnitudes'of theelcments comprised in network 111, 112, 113, the potential of anode 74 thereupon becoming equal to that of terminal 81 with the result that the glowdischarge is re-ignited as has'been described in connection with'Fig. 3.

Itrwill have been noted that in the system shown in- Fig. 4 the voltage across the vessel abruptly jumps to a value lying above the ignition value of the glow discharge atthe instant the arc discharge is extinguished. In this respect, the system according to Fig. 4 differs from that shown in Fig. 3 where the same voltage under the same conditions increases exponentially, as has been explained. Tests have shown that the discharge in the vessel uponre-ignition is less inclined to take the form of an arc discharge if the voltage across the vessel is increasing at a finite rate and that, for this reason, systems operating as the system shown in Fig. 3 docs are preferable. Such" systems may for example be built in accordancewith' Figs. 5 and 6. The circuitry shown in both Figs. 5 and 6 is similarin many respects to that of the systems alreadydescribed, and a brief description will therefore sufiice as far as such similarity exists.

In Fig. 5, a discharge vessel 113 having a cathode 119" and an' anode 120 is connected to the terminals 121and 122 of a D.C. source 123 by leads 124 and 125,1ead124 comprising a resistor 126 and lead comprising a resistor 127. The vessel 118 may be equipped with an ignition electrode 128 conected to the anode 120 across a source 129-and a resistor 130. The anode 131 of an ignitron 132 is connected toanode 120. The mercury tion electrode 128 connected to the anode 120 across aresistor 134 to lead 124in a node 1.35. located between terminal 121 and resistor 126. The igniter 136 of ignitron 132 is connected to the thermionic cathode 137 of a thyratron 138 Whose anode 139 is connected to terminal 122 across a resistor 140, an inductance 141 and a resistor 142; A node 143, located between resistor 142 and-theseries connection of resistor with inductance 141, is connected to lead 124 across a capacitor 144. The resistance of resistor 142'is large as'compared with that of resistor 140. The potential of the grid 145 of thyratron 138 equals the sum of the voltage of a bias voltage source 146' plus the output voltage of a two-loop network- 147, 148, loops 147 and 148 having a capacitor 149 in common. In addition to capacitor 149, loop 147 contains a resistor 150, a capacitor 151 and the resistor 126, whileloop 148; in'additiontocapacitor 149, comprises aresistor 152 connected in series with the bias Voltage source 146. Node 153 of loop 148 is connected to grid 145 across a resistor 154. a

The mercury pool cathode 155 of a second ignitron 156 is connected to lead 124, while its anode 157 is connected to a node 158 which in turn is connected across a resistor 159 to terminal 122 and is also connected across a capacitor 160 to the anode 131 of ignitron 132. The anode 157 of ignitron 156 is further connected to lead 124 across a resistor 161, an inductance 162 and a capacitor 163. The resistance of resistor 159 is large as compared with that of resistor 161. The igniter 164 of ignitron 156 is connected to the thermionic cathode 165 of a thyratron 166 whose anode 167 is connected to terminal 122 of source 123 across a resistor 168, an inductance 169 and a resistor 170, a node 171 located between resistor 170 and the series connection of resistor 168 with inductance 169 being connected to lead 124 across a capacitor 172. A two-loop network 173, 174 in connection with a bias voltage source 175 furnishes the potential of the grid 176 of thyratron 166, loops 173 and 174 having a capacitor 177 as their common element. In addition to capacitor 1 77, loop 173 contains a resistor 178, a capacitor 179 and resistor 134, while loop 174, in addition 'to capacitor 177, contains a resistor 180 in series with the bias voltage source 175. Node 181 of loop 174 is connected to grid 176 across a resistor 182.

Lead 124 and, consequently terminal 121'rnay be assumed to be grounded, as indicated at 183.

In normal operation, with a glow discharge being present between the electrodes 119 and 120 in the vessel 118, ignitrons 132 and 156 as well as thyratrons 138 and 166 are dead. Let now the glow discharge break down to an arc discharge with the result of a sudden increase in the discharge current and, consequently, a corresponding increase in the voltage drop across resistor 126.

This change in the voltage drop across resistor 126 appears in'the form of a delayed voltage. pulse across resistor 152, the delay depending upon the time con stants of the network 147, 148. The voltage pulse across resistor 152 subtracts fronrthe bias voltage of source 146 and, for a brief interval of time, raises the potential of grid 145 sufiiciently to fire thyratron 138. The thyratron 138 while conducting allows capacitor 144 to discharge itself through the igniter 136 and the thercury pool cathode 133 of the ignitron 132, the latter being fired thereby. Thyratron 138, after having delivered the charge of capacitor 144, ceases to conduct, capacitor 144 now being re-charged through resistor 142.

Ignitron 132 when fired causes extinction of the arc discharge in the same way as ignitrons 59 and did with regard to the arc discharges in vessels 50 and 72 in the systems shown in Figs. 3 and 4. At the same time, the current in resistor 134, equalling the ignitron discharge current, causes a voltage pulse to appear across resistor 188 which pulse subtracts from the bias voltage of source 175 and thus raises the potential of grid 176 sufficiently to fire the thyratron 166. Capacitor 172 is now discharged through thyratron 166, igniter 164 and mercury pool cathode of the ignitron 156. The discharge current thus fires the ignitron 156 whereupon the thyratron 166 is automatically extinguished as soon as capacitor 172 is nearly discharged. The latter is then I re-charged to its original voltage.

Comparing Figs. 5 and 3 will make it apparent that the thyratron 138 is equivalent to switch 64 and the ignitron 156 is equivalent to switch 69, and that resistor 159, node 158 and capacitor are the equivalents of resistor67, node 70 and capacitor 68, Hence, the firing of ignitron 156 causes extinction of the discharge in the ignitron 132 in just the same way as the discharge in ignitron 59 was extinguished by the respectively.

closing of switch 69. As soon as the ignitron 132 ceases to conduct the voltage across vessel 118 starts increasing again exponentially until the ignition voltage value of the glow discharge is reached, the glow discharge thus becoming re-ignited, with the voltage across the vessel dropping slightly to the value of normal operation. Ignitr'on 156, too, is extinguished, as'it only conducts as long as capacitor 163 is being discharged, the potential of anode 157 then dropping below a value at which a gas discharge between the anode and the cathode of ignitron 156 is possible.

The inductances 141 and 169 are provided to assist in the extinction of the gas discharges in the thyratrons 138 and 166, and similar provisions will often be found in circuits comprising thyratrons. It should be noted, however, that these inductances, although being of advantage, are not to be considered necessary elements of the circuit. 7

An operation similar to that just described may also be I obtained from a system as shown in Fig. 6. In Fig. 6,

the discharge vessel is designated 184, and it is shown equipped with a cathode 185, an anode 186 and an ignition electrode 187, the latter being connected to anode 186 across a resistor 188 and a source 189. The cathode is connected to the negative terminal 190 of a D.-C. source 191 by means of a lead 192, which lead is grounded at 193. The anode 186 is similarly conto the anode 186 of the vessel 184. The igniter 205 of ignitron 203 is connected to the cathode 206 of a senditron 207, the latter having an anode 208 and an igniter 208a. The anode 208 is connected to terminal 194 across a pairof resistors .209 and 210, a node 221 located between these resistors being connected to ground across a capacitor 212. The resistance of resistor 209 is small as compared with that of resistor 210. Means are provided to apply to the igniter 208a a voltage pulse of high magnitude, these means comprising a transformer 213 whose secondary 214 is on the one hand connected to the igniter 208a and on the .other hand to the anode 216 of a thyratron 217, while the primary 215 is connected on the one hand to anode 216 and on the other hand to the positive terminal of a D.-C. source 219 whose negative terminal is grounded, as is the cathode 218 of thyratron 217. The lead 220 connecting elements 21 5 and 219 comprises a resistor 221 and a node 222,

the latter being connected to It should be noted that the resistance of resistor The potential of the grid 224 of thyratron 217'is a function of the time derivative of the voltage drop across resistor 196, a two-loop network 225, 226 producing a delayed voltage pulse whenever there is a change in the voltage drop across resistor 196. The common element of loops 225 and 226 is a capacitor 227. Loop 225 furcharge, the sudden increase in current in resistor 196 j ground across a capacitor is large as compared with the impedance of primary If the glow '11 and -thecorresponding changedn the voltage-drop across thisresistor will. energize, theanetwork 2255226 withthe result that. a. delayed voltage.- pulse. will appearacro s s. resistor.2 30,rwhich pulse raises the potential of the grid: 224.of-thyratron 217 and thus fires the'latter. Capacitor 223...is.now capable. of discharging itself'through the primary 2l5zof, transformer 213 and therebyinduces a voltage in the secondary 214. The number of turns in the secondary 214 is. so selected that thevoltage across the.,secondary.issufiicient to fire the senditron 207 by raising the potential of the igniter 2tl8a above the firing potential. Senditron 207 when fired causes capacitor.-.212 to-discharge itself-through the igniter 205 and the mer-' cury -pool'zcathode 2020f the ignitron 2tl3, thereby firing the.1atter. Ignitron 203 whenfired causes extinction-of theme-discharge invessel-ltil-in exactly the sameway as -in:the.systems .shown in Figs. 3 to 5. Meanwhile, a

both the thyratron 217 and the senditron 207 are extinguished-as soonasthecapacitors 223 and 212 are nearly discharged and the potentials of anodes-216 and 208 have-dropped below .thevalues'at which gas discharges in-these tubes are possible.

The circuit loop comprising capacitor 291-, resistor 1%, inductance 199-and ignitron 2tl3iisoscillatory due to the presence of capacitance-and inductance, the damping ratio :of apossible oscillation being under-critical because the resistance of resistor-198 can easily be made relatively small. An oscillationis initiated by the closing of-the-said loop through ignitron 203 when the latter fires: The voltage'acrosscapacitor 201 aswell as the loop currentnow begins to change as they do in a damped oscillation with the result that, when the first half-period is completed, the current in the capacitor changes sign. The. gas discharge in the ignitron cannot survivethischange and is therefore extinguished after an interval of time has elapsed which is equal to,-or, to be exact, slightly less-than the half period of the oscillation. During the transient succeeding the extinction of ignitron 203 the capacitor 261 is re-charged, the voltage across thiscapacitor, equallingapproximately the voltage across the vessel 184, now increasing exponentially at thetime constant of the circuit loop containing capacitor 201, resistor 197 and source 191. When finally-the ignition-voltage of the glow discharge is reached the. discharge in vessel 184-is reignited, conditions now being what they were prior to the breakdown.

The graph shown in Fig. 7 will give a better idea of the total process of extinguishing the arc in the discharge 1 vessel 1 and re-establishing the glow discharge. It is assumed in Fig. 7 that'basically operation -is similar to theway the systems shown -in Figs. 3, 5 and 6 operate;v The curve plotted in full lines represents the voltage across the discharge vessel, while the dotted curve; illustrates the discharge current. Both variables areplotted. vs; the time. The graph is to be understood qualitatively only. Y

Assume that normal conditions are present up to an instant designatedA in Fig. 7. Hence, from the origin to- A-the voltage will equal the discharge voltage 1 of the glow discharge, while the current will equal theglow discharge current i Let a breakdown to an are discharge happen at A. The voltage then drops to v,,', equalling the discharge voltage of the arc, while at the same time. the. current increases to the rather high value i,,. A' time interval t now elapses until, at instant B, the ignitron short-circuits the discharge vessel and thus extinguishes the arc. The current now drops to Zero and the voltage becomes equal tofthe discharge voltage; v of the gas dischargein-the ignitron, which discharge voltage is considerably less than v and sufficiently;v small toprohibit an arc discharge from appearing in the vessel. The ignitron is then allowed to conduct for an interval ofqtime t this interval ending-at C wherethe ignitron is-extinguished.- At C thevoltage starts increasingapproximately exponentially until,;at D; aftera time inter- 551 which; makes surethatthe arc does not do any harm,

val. t has elapsed; it reaches the ignition voltage of the glow discharge and thus;gcauses-qrefignition of the dis-. charge in the vessel.

vessel closes the, circuit path through the vessel with'the result that the current again reaches its former valuei while at the same time the voltage drops to value v A plot illustrating the functioning of a system operating ,similarly to that shown in theinstants C and D would coincide and, correspondingly, 1 would equal Zero, explanation given above illCQIlIlBCllOIlQWilh thedescrip tion of Fig. 4.

Certain,recommendations-will now be given as to the-,

suitable length of time of the intervals t t and t It will be recalled that in the examples ofpossible embodiments of the invention described above the intervals .1

and t are functions of networks and thus may be selected within rather wideranges by suitably dimensioning the, network-elements, the method 'to be followed being well;

here that I do not Want to limit the scope of my invention to any particular method or means to produce-time delay. While I usually prefer'to replace the networks 179, and 192; 193, 194in Fig. 3, the networks 147,; 143and 173, 174 in Fig. 5 and the network 225, 22 6 in-Fig.- 6 by suitable electronic means, I often prefer to make use-of-passive linear network elements andcon figurations for-the purpose of controlling the interval t as such elements and configurations have been shown in Figs. 5 and6. Also, the interval of time t if there is any, is preferably obtained by providing a passage linear network, as in Figs. 5 and 6.

It would seem that the time interval t elapsingbetween thebreakdown of the glow discharge to an .-arc discharge and the extinction of the arc, should be made. as short as possible. Whilethis is generallytrue, itwill; often be found that the probability of the discharge to,

re-appear in the vessel in the form of an arc discharge, orto break downto an;arc shortly after the glow dis! charge has beenre-ignited, is reduced if the arc has been allowed to burn for a limited'time.

10- seconds, l0 seconds being a good average value inside, the vessebeven under rather unfavorable, conditions.

As to-the interval t this is the time, during which; the vessel is dead and de-ionization of the discharge space between;the electrodesof the vessel may take, place. I: have foundthat this interval should preferably, lie be-. tween -10 and 10- seconds, in order to obtain-sufficient derionization without extending this interval to an amount which-would make operation rather poor from aneconomical point of view.

The interval t may be relatively short. While there; are cases'in which this time interval may be zero and,; consequently, a system according to Fig. 4 may for ex-. ample be used without-any major disadvantage, it will be:mostly found-preferable to have a t of at least-10' seconds; and I have found that sometimes values as high as .10 seconds will 'give optimum efficiency.

and/or empirical methods the valuesand magnitudesof;

the circuit .;elemen ts that will put the three time,intervals,-

Re-igniting the discharge in the- Fig. 4 would be identical with the plot shown in Fig. 7' with the exception that as-;.will be clear from the.

I should like toemphasize;

While the-upper lunitof. thistimeis sometimes as high as 10- seconds,; I prefer to let the arc burn for a time between 10- and a,sii,576

. 13 within the limits given above. values will be listed here for a system built in accordance with FigIS, which values will ensure that 2. =lseconds, z;;=3.10- seconds and t seconds. These values are the following:

Neverthless, suitable Resistors:

Numeral 126 ohms.. .10- Numeral 127 do 2 Numeral 130 do .10 Numeral 134 do 10 Numeral 140 do. 2 Numeral 142 do 10 Numeral 150 do' l0 Numeral 152 do 10 Numeral 154 do 10 Numeral 159 ..do 10 Numeral 161 ohm 1 Numeral 168 ohms 2 Numeral 170 do 10 Numeral 178 do 5.10 Numeral 180 do 10 Numeral 182 .do 10 Capacitors:

Numeral 144 farads 4.10" Numeral 149 do 10* Numeral 151 do 10- Numeral 168 do 10- Numeral 172 do 4.10

Numeral 177 -1 do 5.10-5

Numeral 179 do 10- Inductances:

'- Numeral 141 henrys 10- Numeral 169 do 10- Biasjvoltages:

. Numeral 146 volts 25 w Numeral 175 do 28 The bias voltages should be adjustable.

The above values apply in a case where the main source of energy 123 produces a voltage between its terminals of 800 volts and, at normal operation, supplies a glow discharge current of 100 amperes at a discharge voltage between the electrodes of the vessel of 600 volts. Whenthe glow discharge breaks down to an arc discharge, the latter voltage will then drop to 50 volts and the discharge current will climb to 375 amperes.

Similar data will also be given for a system built according to Fig. 6 under the assumption that the glow discharge is to be operated at 500 volts and 50 amperes and that the main source of electric energy produces 800 volts. 7

Resistorsz" Numeral 188;-.. oh1'ns 10 Numeral 196 do 10- Numeral 197 do 6 Numeral 198 do 210* *eNumeral 209 do 4 Numeral 210 do 500 Numeral 221 .do 2500 Numeral 228 do 5.10 Numeral 230 do 10 Numeral 233 -do 5000 Capacitors: .Il I

, Numeral .201 farads 10- Numeral 212 do 410- Numeral 2231;; do 75.10- Numeral 227 do 10' Numeral 229 do 10" Inductance: I

Numeral 199 henrys 2.10 Bias voltage:

Numeral 231 volts 8 The bias voltage should be adjustable. The transformer 213 should be layed out to produce a voltage between 5 in the electrical conditions present'in the circuit, these changes'resulting directly or indirectly from the breakdown. With regard to the effect of the breakdown upon the electrical conditions in the circuit the primary etfect is the sudden drop in the voltage across the discharge vessel, whereas the resulting increase in discharge current is a secondary effect.v It will be obvious to any one skilled in the art that there are many feasible and wellknown ways to utilize directly or indirectly either the change in voltage or the'c-hange in current for initiating the automatic operation of the system. It is for reasons of simplicity that the systems illustrated in the wiring diagrams shown in Figs. 4 to 6 the change in current is made use of to start the system.

What I claim is: a a

1. In a system for operating a glow discharge at a discharge current exceeding the order of magnitude of one ampere, in combination, a discharge vessel, means to evacuate said vessel, at least two electrodes inside said vessel, a sou'rceof electric energy, an electric circuit connecting said source and said electrodes and capable of supplying to said electrodes a voltage and a current sufficient for operating said glow discharge, a gas discharge relay tube having an anode, a cathode and an igniter, said tube being capable of producing when conducting a voltage between said cathodeand said anode which is less than the 'voltage across said vessel at which an arc discharge can exist in said vessel, a first loop in said circuit and comprising said source, said anode and said cathode, a second loop in said circuit and comprising said vessel, said anode and said cathode, means adapted to automatically fire a gas discharge in said tube and thereby to reduce the voltage across said vessel to a value less than the discharge voltage of an arc discharge whenever said glow discharge in said vessel breaks down to an arc discharge, and automatically operative means adapted to subsequently re-extinguish said gas discharge in said tube and thereby to raise the voltage across said vessel to a value at which a glow discharge is re-ignited in said vessel.

'2. A system as claimed in claim 1 in which the gas discharge in said relay tube is a mercury vapor type of a discharge.

3. A system as claimed in claim 1,-in which said cathode of said tube is of the mercury pool type.

4."A system as claimed in claim 1, in which said tube" tube being capable of producing when conducting a" voltage between said cathode and said anode which is less than the voltage across said vessel at which an arc discharge can existin said vessel, a first loop in said circuit'and comprising said source, said anode and said cathode, a second loop in said circuit and comprising said vessel, said anode and said cathode, means adapted to automatically fire a gas discharge in said tube and thereby to reduce the voltage across said vessel to a value less than the discharge voltage of an arc discharge in said.

vessel whenever said glow discharge in said vessel breaks down to an arc discharge, automatically operative means adapted to subsequently re-extinguish said gas discharge 7. In a system for operating a glow discharge at a) discharge current;e xceeding the ordero fi-rnagnitude of. one ampere, in combination, a discharge vessel,- means to. evacuate said vessel, at least two electrodes inside said; vessel, a sourceof electric energy, an electric circuit con; necting said source and said electrodes and capable of, supplying to said electrodes a voltageand acurrent sufficient for operating said glow discharge, a gas discharge, relay tube having an anode, a cathode and an igniter, said tube being capable of producing when conducting a voltage between said cathode and said anode which, is less than the voltage across said. vesselatwhich an arc discharge can exist in said vessel, a first loop, in v said ci r cuit and comprising said source, said anode. andrsaid cathode, a second loop in said circuit-andcomprising said vessel, said anode and saidcathode, means adaptedto automatically fire a gas discharge in said tube and-there by;to reduce the voltage across saidvesselto a value less than the discharge voltageof an arc discharge, in said; vessel whenever said glow discharge in said vesselbreaks down to an arc discharge, automatically operative-means adapted to subsequently re-extinguish saidgas discharge; in said tube andthereby to raise the voltage across said vesselto a valueat which a glow discharge isre-ignited insaid vessel, means adapted to delay saidre-extinguish ing bya predetermined length of time, and means adapted, torcontrol the rate at which said voltage acrosssaid vessel is raised after said gas discharge is extinguished.

8, A system as claimed inclaim 7, inwhichsaid rate is such that the interval of timeelapsing from the extinguishingof said gas discharge in said tube to the reigniting of a glow discharge in said vessel is between theorders of magnitude of 107 and 10- seconds,

9. In asystem for operating a glow discharge at a discharge current exceeding the order ofmagnitude of; onetampere, in combination, a discharge vessel, means to evacuate said vessel, at least two electrodes inside said vessel, a source of electric energy, an electriccircuit connecting said source and said electrodes and capableof, supplying to said electrodes a voltage anda current sufficient for operating said glow discharge, a gas discharge relay tube having an anode, a cathode and an igniter, saidtube being capable of producing when conducting a voltage between said cathode and said anode which is lessthan the voltage across said vessel at which an arc discharge can exist in said vessel, a first loop in said circuit and compn'sing said source, said anode and said cathode, a second loop in said circuit and comprising said vessel, said anode and said cathode, means adapted to automatically fire a gas discharge in said tube and thereby to reduce the voltage across said vessel toa value less than the discharge voltage of an arc discharge in said, vessel whenever said glow discharge in said vessel breaks down to an arc discharge, means adapted to de lay said firing of a gas discharge in said tube by a predetermined length of time, automatically operative means adapted "to subsequently re-extinguish said gas discharge in said tube and thereby to raise the voltage across said vessel to a value at which a glow discharge is re-ignited in said vessel, and means adapted to delay saidre-extinguishing by a predetermined length of time.

10,-: At ystemj as l m d n c m 9,, which sa 1 .1 t rmi edmm o meh c id firing ra-areas. discharge in said tube is .delayed is between the orders fia, magnitude. 02 0'? d 7 econd 11. In a system for operating a glow discharge at .a: discharge current exceeding the order of magnitude .of one ampere, in combination, a discharge vessel; to evacuate said vessel, at least two electrodes inside said vessel, a source of electric energy, an electric circuit connecting said source and-saidelectrodes andgcapab le of supplying to said electrodes a voltage and a cu'r'rer'it' sufficient for operating said'glow discharge, a gasdi scharge relay tube having an anode, a cathode'andan'igniter, said tube being capable of producing when conducting a voltage between said cathode and said a ede'wmeh is less than the voltage across saidvessel at which an arc discharge can exist in said vessel, a first loop 'in' said circuit and comprising said-source, said anode andsaid cathode, a second loop in said circuit-and comprisingsaid vessel, saidanode andsaid-cathode, means :adapted to automatically fire a gas discharge in said tube and'thereby to reduce the voltage across'said vessel we value less than the discharge voltage ofanarc discharge in said vessel whenever said glow discharge in said vessel breaks down to an arc discharge, means adapted to'diay said firing ofa'gas dischargc in said tube b ya predetermined length of time, automatically operative means adapted to subsequently reextinguishsaidgas discharge in said tube and-thereby to raise the voltagefacross said ,vessel to a value at which a glow discharge is re-ignited. in'said vessel, means adapted-to delay said r'e-extinguishing by a predetermined length of time, and means adapted to controlthe rate at which-said-voltageacross said vessel is raised after said gas dischargeisiextinguished 12. In a system for operating a glow discharge attadischarge current exceedingv the order. ofmagnitude? of one-ampere, in combination, a discharge vessel, means to evacuate said vessel, at least. two electrodesinside said vessel, a source of. electric energy, an electric circuit corinecting said sourceand s ai d electrode's andcap'able'of, supplying to said eleetrodes fa voltageand a current 'sfuf fici ent for operatingsaidglowdischarge, agasdischarg e relay tube having an anode, a 'cathod'e, andan igniter; said tube being capable of" producing when conducting fa: voltagevb'etween said cathode and said, anode whiehfis} less than the voltage across, saidvessel at which anare discharge can exist. in said, vessel, a, first loop 'infsaid' circuit: and comprising,said source, said anode and said cathode, a second loop in said circuit and: comprjsi saidvessel, said anode and said cathede, means sensitive to a change in the electrical conditions in said circu i' resulting from a breakdown of said glow discharge to an" arc discharge and adapted to automatically fire a, gasw discharge in-said tube andtherebyto. reduce the voltage across said vessel: to a. value lessrthanthedischarge.voltage of an arc-discharge insaidvessel wheneversaidzgl'ow discharge in said vesselbreaksdown to. anarc'. discharge, and automatically. operative vmeans adapted? to. sub'sequently re-extinguish said gas discharge in. said tuheand thereby to raise the voltage. across said vessel tow a, value at which av glow discharge is re-ignited in. said; vessel.

er ms C ted in he e of this, Pa en UNITED STATES PATENTS 2,201,873 Verse May 21,1940 2,623,204 Solomon Dec. 23, 19 52 

